Buy the toy for him.

I tell my kids if they want something, tell me. We'll see if we can buy it. If they ask someone else like relatives or friends to buy it for them, I won't buy anything for them any more. So they always tell me what they need. I let me choose one or two from their list so they know they can't get everything they need.

You may try this way too.

Do tell him that he is a thief. He is just young and naive. Tell him the consequences.

And tell him that he can tell you what he really likes and you would buy it for him.

If he steals again, you will buy nothing for him. He will need to steal everything he likes.